The present invention relates to an optical memory medium wherein high density recording is performed on the surface of a substrate through optically produced chemical changes on the surface of the substrate, to an optical recording apparatus having a microscopic aperture for performing optical recording with high density and to an apparatus for reading information from changes in the frictional force on the surface of the memory medium.
Conventional high density memory media include magnetic disks, optical disks, magneto-optical disks and the like. A magnetic disk is a flat plate applied with a magnetic substance and is written and read by a magnetic head. Since a magnetic disk is magnetized in a horizontal direction, there is a need for preventing magnetic fields from overlapping each other, which need has imposed a certain limit on efforts to obtain higher density. Optical disks and magneto-optical disks were developed in an attempt to allow recording with higher density. In the case of an optical disk, a raw disk is made by means of exposure to laser beams; a master disk is made by means of electroforming from the raw disk; and a replica disk is made by means of injection molding from the master disk. Reading is performed using variations in the reflectance of laser beams which depends on each bit. A magneto-optical disk is obtained by forming a thin film of a vertically magnetized material on a flat plate formed with pits for tracks. To perform writing on the magneto-optical disk, vertical magnetized spots are written into the disk by applying magnetic fields thereto in a vertical direction, with heating of spots irradiated by laser beams maintained above the Curie-point. Reading is performed by directing laser beams and utilizing changes in the polarization angle of the reflected laser beams which depend on the direction of the magnetization of the spots.
Laser beams are used to read bits in both of optical disks and magneto-optical disks which are high density optical memory media. The bit size is limited by the diameter of a spot to which a laser beam is converged and the minimum size is on the order of the wavelength of light, i.e., 1 .mu.m.